1. Field of the Invention
The present invention relates to a chair (rocking chair) provided with a backrest which tilts backward and a support mechanism unit which is used thereof.
2. Description of the Related Art
In a rocking chair, whose backrest tilts backward, a spring means is used to support the backward tilt of the backrest. The spring means includes a compression coil spring, blade spring, torsion bar, rubber, gas cylinder (or gas spring) and the like, and in general, a compression coil spring is commonly used. The compression coil spring is in most cases arranged in a box-shaped base opened upward in a posture extending approximately in a back and forth direction, when viewed from above.
Further, the coil spring is supported by front and rear spring bearings, and a space between these front and rear spring bearings adjusts an initial elastic force (hardness) of the coil spring. The initial elastic force of the coil spring often makes use of an adjustment mechanism by the use of a wedge member to change a position of either the front spring bearing or the rear spring bearing. A handle for operating the wedge member (it may be called a knob or grip) is often arranged on a lower surface site of the base.
In addition, the backrest is often attached to a back support unit attached to the base so as to tilt backward, and in general a compression coil spring is contracted or deformed by the backward tilt of the back support unit.
A rocking mechanism in which the above-described compression coil spring is used has been disclosed by Japanese Published Examined Patent Application No. Sho-48-37430 and Japanese Published Examined Utility Model Application No. Sho-48-40818. More specifically, the rocking mechanism is constructed in such a way that a projecting portion projecting behind a seat is provided on a base, a back support unit is attached to the projecting portion by using pins so as to tilt backward, a cylinder opened backward in a forward tilting posture when viewed from the side is fixed to the base, a compression coil spring and an adjusting member for pushing the coil spring are fitted inside the cylinder, by which a load resulting from the backward tilt of the back support unit is allowed to act on the compression coil spring from behind via the adjusting member.
In the chair disclosed by the above two publications and many other chairs, since a pressing force is allowed to act upon a compression coil spring at a site further forward than a tilt supporting point (turnable supporting point) of a back support unit, the compression coil spring undergoes a great load due to the leverage, while at the same time, the compression coil spring is restricted in length due to a limited space inside the base. Therefore, conventionally, a compression coil spring is made with a thick wire to secure a necessary repulsive force (elastic restoring force), even if the coil spring is short in length.
On the other hand, it is known that where a seated person reclines against a backrest to assume a rocking posture, he or she may have a sense of discomfort due to the impact, upon an abrupt backward tilt of the backrest, however, where the backrest tilts backward slowly all the way to the backward tilt limit, he or she does not have a sense of discomfort but feels comfortable. Further, when a seated person sits up from a rocking motion, it is preferable that a backrest moves accordingly and slowly.
However, where the spring is kept in a certain posture as disclosed in the publications, a problem occurs when either a backrest abruptly tilts backward for a very short time when a seated person reclines against the backrest, or only a slight backward tilt does not produce a backward tilt due to an abruptly increased repulsive force of a spring, thereby making it impossible to provide a comfortable rocking motion.
This point may be solved by increasing an initial elastic force of a compression coil spring. However, a greater initial elastic force of the compression coil spring only results in a difficulty in backward tilt, and no comfort is, therefore, expected when a greater initial elastic force is imparted to the compression coil spring.
Further, if a maximum backward tilt angle of a backrest is made greater, a comfortable rocking motion can be obtained. However, there has been a problem in that a conventional spring is restricted in length and a backward tilt angle of the backrest is restricted accordingly, making it difficult to increase a backward tilt angle of the backrest. In addition, another problem occurs where a handle for adjusting an initial elastic force of a spring is arranged on a lower surface of a base, a person has to operate the handle in a bent posture, therefore, making the adjusting operation more troublesome.
On the other hand, various structures are adopted in a backrest of a chair. In a revolving chair commonly used in an office (a chair whose seat is supported by a leg pillar so as to rotate horizontally), a backrest is in most cases structured with three layers, namely, a resin-made outer shell functioning as a reinforcing member, a resin-made inner shell functioning to support the body and a cushion layer applied on a front surface of the inner shell, and the outer shell is often fixed to the back support unit by using screws.
The conventional three-layer structured backrest is advantageous in excellent cushioning characteristics. However, there is one problem that workability is not always good in assembling of chairs. More specifically, in a chair whose backrest has a conventional three-layered structure, a cushion is in general attached to an inner shell in advance, and after attachment of an outer shell to a back support unit, the inner shell is attached to the outer shell. It may take more time and labor to assemble the chair, because the attachment of the outer shell to the back support unit and the attachment of the inner shell to the outer shell must be performed.
In a case of assembly by a knock-down method where several unitized members are assembled, it is desirable that assembly processes should be reduced as much as possible in view of workability and stable quality. It is, therefore, required to reduce the number of components as much as possible, for which a conventional backrest structure will not satisfy the requirement.
There is also a problem related to the conventional three-layer structured backrest that only an outer shell does not function as a backrest and always needs a cushion-equipped inner shell, which makes it difficult to simplify the structure. In other words, a conventional outer shell is in general provided with many ribs formed on the front surface for reinforcement. Therefore, a person is not allowed to recline against the outer shell, thus making it impossible to simplify the structure of the backrest.